Effects of Superfine Cement on Fluidity, Strength, and Pore Structure of Superfine Tailings Cemented Paste Backfill

Abstract

Advancements in mine tailings treatment technology have increased the use of superfine tailings, but their extremely fine particle size and high specific surface area limit the performance of superfine tailings cemented paste backfill (STCPB). This study investigates the effects of using superfine cement as a binder to enhance the fluidity, strength, and pore structure of STCPB. The influence of water film thickness (WFT) on STCPB performance is also examined. The results show that the cement-to-tailings ratio (CTR) and solid content (SC) significantly affect the spread diameter (SD) and unconfined compressive strength (UCS), following distinct linear/logarithmic and exponential trends, respectively. WFT has an exponential impact on SD and a non-linear effect on UCS, enhancing strength at low levels (0 μm < WFT < 0.0071 μm) and balancing hydration and flowability at moderate levels (0.0071 μm < WFT < 0.0193 μm) but reducing strength at high levels (WFT > 0.0193 μm). Additionally, superfine cement significantly improves the pore structure of STCPB by reducing porosity and macropore content. These findings provide valuable insights into optimizing STCPB for enhanced performance and sustainability in mine backfilling applications.

1. Introduction

Cemented paste backfill (CPB) is a vital underground mining technology extensively employed to stabilize mined-out voids and promote the sustainable management of mine tailings [1,2,3,4]. CPB typically comprises mine tailings, binders (such as cement), and water, forming a mixture that fills underground voids to enhance structural stability and minimize the risk of collapse [5,6,7]. One of CPB’s key advantages is its efficient use of tailings, which mitigates environmental challenges associated with tailings storage while enhancing the safety and efficiency of mining operations [8,9,10]. Additionally, CPB delivers numerous benefits, including optimized resource utilization and a reduced environmental footprint, establishing it as a cornerstone of modern sustainable mining practices [11,12,13].

Mine tailings are a fundamental component of CPB, typically constituting more than 66% of its total mass [14]. Given their substantial proportion, the physical and chemical properties of tailings play a critical role in determining the mechanical and rheological behavior of CPB. Advances in crushing technology and extraction processes have significantly altered the particle size distribution of tailings, increasing the proportion of superfine particles and giving rise to superfine tailings cemented paste backfill (STCPB) [15,16]. While STCPB offers certain advantages for potential applications, the high specific surface area of superfine tailings leads to excessive water demand and limited strength development. These limitations adversely affect the transportability and mechanical strength of STCPB, thereby restricting its application in underground support engineering [17]. As a result, improving the performance of STCPB has become a primary research focus within the field of mine backfilling technology.

In recent years, researchers have explored various strategies to enhance the performance of STCPB. These approaches include optimizing mix proportions, incorporating supplementary cementitious materials (such as fly ash and slag), and utilizing chemical additives. Wang et al. [18] demonstrated that optimizing the proportions of blast furnace slag, carbide slag, and additives (sodium silicate and calcium chloride) significantly improved the setting and strength properties of STCPB. Guo et al. [17] developed a novel blended binder comprising high-volume blast furnace slag and a superplasticizer, which effectively enhanced STCPB’s performance. Zhang et al. [19] found that adding Al_2-O_3- reduced STCPB porosity, thereby increasing its strength. Similarly, Hu et al. [20] reported a significant improvement in the early strength of STCPB through the use of nano-SiO_2-. Despite these advancements, practical applications face several challenges, including high costs, limited improvements in certain performance areas, and inadequate long-term stability. Furthermore, many strategies encounter obstacles related to economic feasibility, environmental sustainability, and limited understanding of their effects on microstructural mechanisms. As such, the development of cost-effective and environmentally friendly solutions to comprehensively enhance STCPB performance remains a critical research priority.

Research has demonstrated that superfine cement offers significant advantages in enhancing the performance of cement-based materials. For instance, Shi et al. [21] reported that the high specific surface area of superfine cement markedly improves the early strength of materials. Similarly, Chen and Kwan [22] found that superfine cement enhances material density and durability, thereby increasing compressive strength and long-term stability. The smaller particle size and increased specific surface area of superfine cement provide distinct technical benefits. Specifically, the reduced particle size enhances particle packing density and accelerates hydration reactions, strengthening the bond between tailings and hydration products [23]. Furthermore, the high reactivity of superfine cement reduces porosity, significantly improving material density and compressive strength [24]. Consequently, replacing conventional cement with superfine cement shows substantial potential to improve both the early and long-term strength of STCPB. Additionally, superfine cement reduces the required amount of binding material, thereby lowering production costs and carbon emissions and offering considerable economic and environmental benefits [25]. These advantages underscore the broad application potential of superfine cement in optimizing STCPB performance, establishing it as an effective solution to address engineering and environmental challenges.

This study employed superfine cement as binder to prepare STCPB and systematically evaluated its flowability, mechanical properties, and microstructural characteristics, with the objective of providing scientific guidance for optimizing STCPB performance.

2. Raw Materials and Sample Preparation

The STCPB utilized in this study comprises superfine tailings, superfine cement, and deionized water. The superfine tailings were sourced from a mine in Liaoning Province, China, exhibit a narrow particle size distribution with a particle size range of 0.2–100 μm, and were 80% finer than 20 μm. Detailed particle size distribution data are presented in Figure 1. X-ray fluorescence (XRF) analysis reveals that the primary chemical components of the superfine tailings were SiO₂ (approximately 58%), Al₂O₃ (approximately 14%), and K₂O (approximately 7%), along with trace amounts of CaO, MgO, and other oxides. The superfine cement was prepared by grinding 42.5 ordinary Portland cement (OPC), with its main components being CaO (approximately 65%), SiO₂ (approximately 20%), Al₂O₃ (approximately 5%), and minor quantities of MgO and Fe₂O₃. The particle size distribution of superfine cement closely matched that of the superfine tailings, with a fineness reaching 85%. Figure 1 illustrates the comparative particle size distributions of superfine cement and 42.5 OPC. Deionized water was employed to eliminate impurities, ensuring that the mixture remained uncontaminated and preventing interference with the experimental results.

During sample preparation, superfine tailings, superfine cement, and water were accurately weighed based on the predetermined mass ratio. The superfine tailings and superfine cement were thoroughly mixed to achieve uniformity. Water was then gradually added while stirring at an appropriate speed for 5 min until a homogeneous STCPB slurry was formed. The resulting STCPB samples were subsequently utilized for further testing.

3. Experimental Methods

3.1. Fluidity Test

In this experiment, the fluidity of fresh STCPB was assessed by measuring the spread diameter (SD) of the slurry. A conical metal mold with a base diameter of 100 mm, a top diameter of 50 mm, and a height of 150 mm served as the experimental apparatus. This choice improves measurement sensitivity and aligns with similar studies on paste backfill materials [26]. The slurry was thoroughly mixed to ensure uniformity and promptly filled into the mold. During filling, it was compacted to eliminate air entrainment and ensure consistent density. The mold was then carefully removed, and the maximum diameter of the spread slurry on the reference surface was measured and recorded as the SD. To ensure the reliability and repeatability of the results, each sample was tested independently at least three times, with the average value reported as the final result.

3.2. Unconfined Compressive Strength (UCS) Test

The compressive performance of STCPB was assessed through the UCS test [27]. Cylindrical specimens with a base diameter of 50 mm and a height of 100 mm were used. The tests were performed using a Humboldt HM-5030 uniaxial compression testing machine(manufactured by Humboldt Mfg. Co., Elgin, IL, USA), with a loading rate of 1 mm/min applied until specimen failure [17]. During testing, load and deformation were recorded in real time, and a load–displacement curve was generated. The UCS was calculated as the ratio of the maximum load to the specimen’s cross-sectional area. Each specimen type was tested at least three times, and the average value was reported as the final result.

3.3. Packing Density and Water Film Thickness (WFT) Test

The wet packing test is an experimental method used to assess the packing density of particulate materials [28]. This approach evaluates the packing behavior and variations in solid concentration by gradually adjusting the water content. During the initial stages of water addition, liquid bridge forces between particles increase, reducing inter-particle distances and thereby raising the solid concentration [26]. Once the maximum solid concentration is achieved, further water addition causes particle dispersion, increases the slurry volume, and decreases the solid concentration. The maximum solid concentration is defined as the packing density ($\varnothing$) of the system.

After determining the packing density, WFT can be calculated using the following formula [29]:

$$WFT={\displaystyle \frac{v_w-v_v}{{SSA}_p}}$$

(1)

where $v_w$ represents the ratio of the system’s water volume to the particle volume, while $v_v=(1-\varnothing )/\varnothing$ denotes the voids ratio. ${SSA}_p$ refers to the specific surface area of the system’s particles.

3.4. Mercury Intrusion Porosimetry (MIP) Test

This study utilized the MIP test to analyze the pore structure of STCPB. The Auto-Pore IV 9510 mercury intrusion porosimeter was used to measure the volume changes caused by mercury intrusion under varying pressures. This method allows for the evaluation of key parameters such as porosity, pore size distribution, and pore connectivity. After the STCPB sample was dried to a constant weight, it was placed in the sample chamber, where pressure was gradually applied. The pore volume was then calculated using the Washburn equation, and the pore size distribution curve was generated.

4. Results and Discussion

4.1. Fluidity Characteristics of STCPB

Table 1. SD of STCPB under different SC and CTR (cm).

SC (%)	CTR
	1:10	1:8	1:6	1:4
64	37.68	35.74	34.16	31.59
66	30.08	28.96	27.77	25.14
68	25.73	24.73	23.80	22.16
70	23.39	22.82	21.94	20.63

presents the SD values of fresh STCPB at different proportions. A higher SD value indicates better fluidity, while a lower SD value corresponds to poorer fluidity [26]. The cement–tailings ratio (CTR) and solid content (SC) are two key factors influencing the fluidity of CPB. To explore the quantitative relationship between the SD of STCPB and both the SC and CTR, one factor was kept constant while the other was varied [30]. Furthermore, to investigate the underlying mechanism of fluidity in STCPB, WFT was introduced, and its relationship with the SD was analyzed.

4.1.1. Effect of CTR

The SC was held constant while the CTR was varied to investigate the fluidity of fresh STCPB. The changes in fluidity were quantified by fitting the data using linear, exponential, and logarithmic models, and the corresponding correlation coefficients (R²) were calculated. The results are presented in

Table 2. Fluidity fitting results of STCPB samples under different CTRs.

Fitting Type	SC (%)				Average
	64	66	68	70
Linear	0.991	0.951	0.981	0.967	0.973
Exponential	0.986	0.940	0.975	0.961	0.966
Logarithmic	0.930	0.844	0.902	0.864	0.885

. The SD scatter plots and linear fitting of STCPB with varying SC and CTRs are shown in Figure 2.

As observed in Table 2, both the linear and exponential fittings exhibit high degrees of correlation, with correlation coefficient values of 0.973 and 0.966, respectively. In contrast, the logarithmic fitting yielded a lower correlation coefficient of 0.885. Therefore, a clear linear relationship exists between the CTR and SD of fresh STCPB, as illustrated in Figure 2. This suggests that a higher cement content impairs the fluidity of STCPB, likely due to the increased generation of hydration products, such as calcium silicate hydrate (C-S-H), making it more difficult for particles to slide past each other [2]. A similar conclusion was also drawn by Cao et al. [31]. The relationship between the CTR and SD of STCPB can be expressed by the following equation:

$$y=A_{1}x+B_1$$

(2)

where $y$ represents the SD of STCPB, and $x$ is the CTR. $A_1$ and $B_1$ represent the fitting parameters related to the SC.

4.1.2. Effect of SC

The SC plays a critical role in determining the fluidity of fresh STCPB. As shown in

Table 3. Fluidity fitting results of STCPB samples under different SC.

Fitting Type	CTR				Average
	1:4	1:6	1:8	1:10
Linear	0.912	0.942	0.940	0.941	0.934
Exponential	0.943	0.969	0.968	0.970	0.962
Logarithmic	0.990	0.997	0.996	0.998	0.995

, increasing the SC from 64% to 70% leads to a consistent decrease in the SD across all CTRs (Figure 3). This behavior is attributed to the reduction in free water content at higher SC levels, which lowers fluidity by limiting the movement of particles [32,33]. To more effectively investigate how the SD of STCPB samples changes with varying SC, the CTR was held constant. Linear, exponential, and logarithmic fittings were applied to the SD data, and the corresponding correlation coefficients are summarized in Table 3. It is apparent that all fitted correlation coefficients are relatively high, with all exceeding 0.9. The correlation coefficient for the logarithmic fit is the highest, reaching 0.995. Thus, the relationship between the SC and SD of STCPB is best described by a logarithmic equation. The corresponding expression is as follows:

$$y=A_{2}ln\left(x\right)+B_2$$

(3)

where $y$ represents the SD of STCPB, and $x$ is the SC. $A_2$ and $B_2$ represent the fitting parameters related to the CTR.

4.1.3. Relationship Between WFT and Fluidity of STCPB

To further investigate the mechanism underlying the fluidity of STCPB, WFT was introduced. Previous studies [17,34,35] have established that WFT is a critical factor influencing the fluidity of CPB. However, these studies primarily focus on conventional CPB, highlighting the need to explore the quantitative relationship between WFT and the SD of STCPB. The results are presented in Figure 4, which clearly illustrates a positive and significant correlation between WFT and the SD of STCPB. Specifically, as the WFT increases from 0.001 to 0.041 µm, the SD increases from 20.63 to 37.68 cm. Regression analysis reveals a strong exponential relationship between the WFT and SD, with a correlation coefficient of 0.99. This indicates that an increase in WFT leads to a corresponding increase in the SD of STCPB, suggesting an enhancement in the fluidity of the material. This behavior can be attributed to the mechanism whereby a thicker water film layer reduces inter-particle friction, allowing particles to move more freely, thereby improving fluidity [36]. A similar relationship between the SD and WFT was observed by Qiu et al. [14] when studying traditional CPB. Thus, for STCPB, as with conventional CPB, WFT is also a key factor influencing fluidity.

4.2. UCS Characteristics of STCPB

Table 4. UCS of STCPB samples (MPa).

CTR	Curing Time (d)	SC (%)
		64	66	68	70
1:4	3	0.60	0.67	0.76	0.86
	7	0.82	0.89	1.09	1.32
	14	1.08	1.19	1.31	1.62
	28	1.39	1.52	1.84	2.37
1:6	3	0.48	0.54	0.61	0.76
	7	0.68	0.74	0.85	1.04
	14	0.91	0.97	1.06	1.33
	28	1.12	1.24	1.38	1.76
1:8	3	0.45	0.49	0.55	0.67
	7	0.61	0.67	0.76	0.95
	14	0.81	0.88	0.95	1.28
	28	1.01	1.12	1.24	1.61
1:10	3	0.41	0.45	0.51	0.63
	7	0.56	0.63	0.71	0.89
	14	0.78	0.82	0.89	1.14
	28	0.95	1.05	1.15	1.51

presents the UCS values of STCPB at various proportions. The CTR and SC are considered the two primary factors influencing the UCS of backfill materials. To investigate the quantitative relationships between the UCS of STCPB and these two factors, a systematic approach was employed in which one factor was varied while the other was held constant. This method was adopted to eliminate the potential effects of factor interactions and isolate the influence of each individual factor on UCS.

4.2.1. Effect of CTR

The SC was held constant while the CTR was varied to investigate the UCS of hardened STCPB. To quantify the variations in UCS, linear, exponential, and logarithmic fitting models were applied, and the corresponding correlation coefficients were calculated. The results are presented in

Table 5. Fitting results of STCPB samples under different CTRs.

Curing Time (d)	Fitting Type	SC (%)				Average
		64	66	68	70
3	Linear	0.914	0.909	0.913	0.968	0.926
	Exponential	0.940	0.938	0.942	0.981	0.951
	Logarithmic	0.791	0.775	0.780	0.863	0.802
7	Linear	0.938	0.914	0.892	0.870	0.903
	Exponential	0.961	0.939	0.926	0.904	0.933
	Logarithmic	0.820	0.783	0.752	0.724	0.770
14	Linear	0.910	0.919	0.915	0.905	0.912
	Exponential	0.936	0.946	0.943	0.924	0.937
	Logarithmic	0.769	0.789	0.783	0.806	0.787
28	Linear	0.898	0.912	0.870	0.833	0.878
	Exponential	0.928	0.939	0.910	0.876	0.913
	Logarithmic	0.760	0.778	0.724	0.679	0.735

. Figure 5 illustrates the UCS histogram, along with the exponential fitting results for STCPB at various SC and CTRs.

From Figure 5, it is evident that irrespective of curing time, the strength of STCPB exhibits a significant upward trend with increasing CTRs. This can be attributed to the fact that a higher CTR results in an increased cement content, which, in turn, enhances the formation of hydration products. These products not only fill the pores but also contribute to the densification of the STCPB structure, thereby significantly improving its UCS [37]. A comparison of the average correlation coefficients for different fitting functions (Table 5) reveals that the exponential function yields the highest correlation coefficient, suggesting that the exponential model best describes the relationship between the CTR and the strength of STCPB. Consequently, the relationship between the CTR and STCPB strength can be accurately represented by the exponential function, as shown in Equation (4). Qiu et al.’s study [30] on the strength characteristics of cemented superfine unclassified tailings backfill reached similar conclusions, indicating that variations in cement particle size do not alter the mechanism by which the CTR affects STCPB strength.

$$y=A_3{exp}^{B_{3}x}$$

(4)

where $y$ represents the strength of STCPB, and $x$ is the CTR. $A_3$ and $B_3$ are fitting parameters, which are related to the SC and curing time.

4.2.2. Effect of SC

The SC plays a critical role in determining the UCS of hardened CPB. Figure 6 illustrates histograms and corresponding exponential fitting curves that depict the relationship between the UCS of STCPB and the SC under various curing times and CTRs. As shown in the figure, an increase in SC leads to a notable upward trend in the UCS of STCPB. This can be primarily attributed to the fact that a higher SC content significantly reduces the porosity of STCPB, allowing for more compact particle packing and promoting densification [38]. Moreover, hydration products are more effective at filling the voids between particles, further enhancing the internal structural integrity of STCPB [39]. Additionally, an increase in SC is typically accompanied by a reduction in the water-to-cement ratio, which helps minimize excess water, thereby decreasing the volume of residual pores after curing and further improving the overall strength of STCPB.

To further investigate the relationship between the SC and UCS of STCPB, three fitting functions (linear, exponential, and logarithmic) were applied to the data, and the corresponding correlation coefficients are presented in

Table 6. Fitting results of STCPB samples under different SC.

Curing Time (d)	Fitting Type	CTR				Average
		1:10	1:8	1:6	1:4
3	Linear	0.953	0.946	0.946	0.991	0.959
	Exponential	0.975	0.970	0.971	0.998	0.979
	Logarithmic	0.845	0.829	0.832	0.916	0.856
7	Linear	0.935	0.934	0.943	0.954	0.942
	Exponential	0.962	0.961	0.969	0.978	0.968
	Logarithmic	0.820	0.814	0.821	0.836	0.823
14	Linear	0.853	0.855	0.897	0.928	0.883
	Exponential	0.889	0.895	0.926	0.954	0.916
	Logarithmic	0.703	0.710	0.762	0.809	0.746
28	Linear	0.886	0.901	0.922	0.930	0.910
	Exponential	0.922	0.935	0.951	0.965	0.943
	Logarithmic	0.752	0.771	0.798	0.799	0.780

. The results indicate that irrespective of curing time and CTRs, the logarithmic function yielded the lowest correlation coefficient, followed by the linear function, with the exponential function achieving the highest correlation coefficient. This suggests that the exponential function (Equation (5)) provides the best fit to describe the effect of SC on the UCS of STCPB. Consequently, the influence of SC on the UCS of STCPB is most accurately represented by the exponential model. Similar results were observed by Qiu et al. [30]. Thus, despite the significant changes in particle size of superfine cement compared to ordinary cement, this alteration does not modify the fundamental mechanism by which the SC influences the strength of STCPB.

$$y=A_4{exp}^{B_{4}x}$$

(5)

where $y$ represents the strength of STCPB, and $x$ is the SC. $A_4$ and $B_4$ are fitting parameters, which are related to CTRs and curing time.

4.2.3. Relationship Between WFT and UCS of STCPB

Figure 7 shows the relationship between WFT and the UCS of STCPB. It is evident that regardless of curing time, the UCS data points exhibit considerable scatter, and the fitting results based on the logarithmic function yield a low correlation coefficient. This suggests that using WFT as a sole variable is insufficient to fully capture the variation in the UCS of STCPB. The influence of WFT on UCS is complex, involving interactions among multiple factors. Differences in particle morphology, surface characteristics, and hydration processes between superfine cement and superfine tailings cause the impact of WFT on UCS to vary under different conditions. Due to its larger specific surface area, superfine cement interacts more extensively with water, leading to a faster and more complete hydration reaction [40]. In contrast, superfine tailings alter the interaction between cement and tailings particles. In particular, within different WFT ranges, the influence of the water film on cement hydration and particle interactions differs, making it difficult for a single fitting method to fully capture the complex relationship between WFT and UCS.

To gain further insight into this relationship, WFT was divided into three distinct ranges, and separate fitting analyses were performed. The resulting fit shows a significant improvement in the correlation coefficient (Figure 8). This suggests that, within different WFT ranges, there are marked differences in cement hydration and particle interaction mechanisms, which, in turn, influence the strength of STCPB differently. A detailed analysis is presented below:

In the low WFT range (0 μm < WFT < 0.0071 μm), the water film primarily serves to promote the hydration of superfine cement particles and enhance the bonding between particles. Due to their large specific surface area, superfine cement particles interact more extensively with water, resulting in a more thorough hydration reaction. The resulting hydration products (e.g., C-S-H) effectively strengthen the bond between cement and tailings particles, significantly improving the strength of STCPB. Additionally, lower WFT restricts slurry flowability, leading to closer particle contact and enhancing the compactness and strength of the structure. Overall, low WFT significantly improves STCPB strength by promoting cement hydration and enhancing particle cementation.

In the moderate WFT range (0.0071 μm < WFT < 0.0193 μm), the water film has a dual role: it not only facilitates the hydration of cement but also maintains the slurry’s appropriate flowability, preventing excessive viscosity and ensuring good particle dispersion. Under these conditions, cement hydration proceeds fully, and moderate WFT promotes the formation of an optimal bond between cement and tailings particles. The hydration products moderately enhance the bonding strength between particles. Furthermore, optimal WFT ensures the slurry maintains its flowability, promoting a uniform distribution of particles in STCPB while reducing voids and structural defects.

In the high WFT range (WFT > 0.0193 μm), an overly thick water film inhibits the full contact between cement particles and water, significantly reducing hydration efficiency. The reduced contact area between cement and tailings particles results in insufficient hydration product formation, hindering particle bonding and limiting STCPB strength improvement. Additionally, although higher WFT improves particle dispersion, the bonding force between particles weakens, leading to the formation of more voids and structural defects. Therefore, under high WFT conditions, the water film not only limits cement hydration but also hinders effective particle cementation, leading to a substantial decrease in the UCS of STCPB.

4.3. Pore Structure Characteristics of STCPB

Figure 9 presents the pore structure characteristics of STCPB samples containing superfine cement and OPC (denoted as STCPB-1 and STCPB-2, respectively) after 28 days of curing. The CTRs for STCPB-1 and STCPB-2 are 1:10 and 1:6, respectively, with all other mix ratios remaining constant.

As shown in Figure 9a, the porosity of STCPB-1 and STCPB-2 is 43.4% and 47.6%, respectively. Despite the lower content of superfine cement in comparison to OPC, these results indicate that superfine cement still significantly reduces the porosity of STCPB. To further assess the pore structure, the pores are divided into four categories, namely, gel micropores (<10 nm), transition pores (10–100 nm), capillary pores (100–1000 nm), and macropores (>1000 nm), according to [32]. The distribution of these pore types in STCPB-1 and STCPB-2 is illustrated in Figure 9b. Notably, the proportion of macropores in STCPB-1 (60.2%) is significantly lower than that in STCPB-2 (66.4%). This difference suggests that even a small amount of superfine cement can effectively refine the pore structure of STCPB compared to OPC. Therefore, superfine cement significantly optimizes the pore structure of STCPB by reducing both porosity and the proportion of macropores, which, in turn, enhances its mechanical performance. In addition, lower porosity typically reduces permeability, improving resistance to water infiltration and contaminant leaching, which is beneficial for environmental safety and long-term durability [41]. However, overly low permeability may impede pore water pressure dissipation during curing, potentially leading to cracking or instability [42].

4.4. Interaction of STCPB with Reinforcements: Challenges and Characteristics

The distinct features of STCPB, such as its refined pore structure, enhanced hydration reactions, and reduced macropore content due to the superfine cement, have a significant impact on its interaction with reinforcements in underground construction. The reduced porosity and improved hydration product formation enhance the bond strength with steel reinforcements and fibers, contributing to better structural integrity [43]. However, the superfine particle size and high packing density may alter the interaction mechanisms with geosynthetics, potentially reducing the interface shear strength [44]. Similarly, the bonding performance with fibers may depend on the specific material type and its ability to integrate with the dense and cohesive matrix of STCPB [45].

5. Conclusions

This study systematically investigated the influence of superfine cement on the fluidity, strength, and pore structure of superfine tailings cemented paste backfill (STCPB). Additionally, water film thickness (WFT) was introduced as a key parameter to explore the underlying mechanisms governing the observed behaviors. The main conclusions are as follows:

• The cement–tailings ratio (CTR) and solid content (SC) exhibit distinct linear and logarithmic relationships, respectively, with the spread diameter (SD) of STCPB. Furthermore, WFT demonstrates a significant exponential correlation with SD, where an increase in WFT reduces inter-particle friction, thereby enhancing fluidity;
• The CTR and SC demonstrate exponential relationships with the UCS of STCPB, with higher values enhancing strength due to increased hydration products and reduced porosity;
• The effect of WFT on the UCS of STCPB is complex and varies across low, moderate, and high ranges. At low WFT (0 μm < WFT < 0.0071 μm), cement hydration and particle bonding are enhanced, resulting in higher strength. In the moderate WFT range (0.0071 μm < WFT < 0.0193 μm), a balance between hydration and flowability is achieved. However, at high WFT (WFT > 0.0193 μm), hydration efficiency and bonding decrease, leading to a reduction in STCPB strength;
• The introduction of superfine cement significantly improves the pore structure of STCPB, reducing porosity and the proportion of macropores. These structural refinements enhance the overall mechanical performance compared to conventional cement.

While this study provides valuable insights into the performance of STCPB, several limitations remain. Long-term strength and durability testing under real-world conditions were not addressed in this study and are essential for evaluating the material’s behavior over time. Additionally, the economic feasibility and environmental impact of large-scale applications of superfine cement warrant further investigation. Future studies could focus on these aspects to better understand the practical implications of using superfine cement in mine backfilling operations.